Air Coupling

I am trying to learn more about air coupling.
That is the coupling between 2 shafts
I am having a hard time finding any information on the subject matter
does anyone know how these systems work and have any diagrams?
thanks

Something like how when you blow one fan into another fan, the second fan's rotors start moving as well? It seems like a very weak sort of coupling... maybe better in more viscous fluids or the other way around?

Certainly, if one takes an axial flow from one fan and impose it on another fan, the second fan's blade will experience energy/momentum from the flow and start to rotate. That would be expected.

In conventional turbo-compressors and turbines, one normally has a set of stationary blades (stator) and rotating set of blades (rotor). For a very simple turbine or compressor with two stages, one could have counter-rotating blade sets. One can get comparable energy transfer but at reduced rotational speed.

Engine spools do, at times, counter rotate to the other. The rotational directions are set up via gas path angles in the rotors themselves. I have not really heard of this as a coupling. The usual mechanical coupling is a curvic coupling. I'll do some seariching around now that you gave a hint to it's application. It would be nice to see the link you are referring to.